[Bug c++/109870] New: Miscomputation of return type of unevaluated lambda in type alias in template context

[Bug c++/109870] New: Miscomputation of return type of unevaluated lambda in type alias in template context

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109870

            Bug ID: 109870
           Summary: Miscomputation of return type of unevaluated lambda in
                    type alias in template context
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

template<class> struct X {};
template<class T>
struct M {
    using R = decltype([] { return 1; }());
    template<class U> struct S { X<R> p; };
};
M<int>::S<void> s;

9.1 through 14.0.0 trunk reject with:

<source>: In instantiation of 'struct M<int>::S<void>':
<source>:7:17:   required from here
<source>:4:36: error: return-statement with a value, in function returning
'void' [-fpermissive]
    4 |     using R = decltype([] { return 1; }());
      |                                    ^

It appears the `U` type parameter in `M<T>::S<U>` is overwriting the lambda
deduced return type.

Possibly related to #92707, #103569

[Bug c++/109870] Miscomputation of return type of unevaluated lambda in type alias in template context

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109870

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Most likely a dup of a bug that PR 107430 depends on.

[Bug c++/109870] Miscomputation of return type of unevaluated lambda in type alias in template context

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109870

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-04-13
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. still fails.

[Bug c++/109870] Miscomputation of return type of unevaluated lambda in type alias in template context

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109870

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=106221

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Very much related to PR 106221 .